1. Field of the Invention
The present invention relates to a particle analyzer, and more particularly to a particle analyzer for analyzing morphology of particles such as a fine ceramic powder, pigment powder or cosmetic powder.
2. Description of the Related Art
It is extremely important to measure and analyze the size of particles for controlling the quality of a powder such as a fine ceramic powder, pigment powder, cosmetic powder or the like. A conventionally available measuring device uses a liquid sedimentation method or an electrozone method (Coulter principle) whereas a currently known measuring device uses a laser diffraction method.
However, none of measuring devices using the aforementioned methods are satisfactory in terms of precision (accuracy) in measurement and measuring range thereof. In particular, when a particle to be measured has a flat or elongated shape, the particle diameter to be measured will be largely different depending on the measuring method. Besides, even when the same measuring method is adopted, measured values are largely different and peculiar particle size distribution curves are given depending on the type of the measuring device.
Since larger particles sediment faster than smaller particles in a suspension, the concentration of particles varies with the lapse of time and in three demensions. A method for determining a particle size distribution by detecting the particle concentration based on an amount of light transmitted though the suspension is a liquid sedimentation light transmitting method, which is typical as the liquid sedimentation method. A device using this method has the following drawbacks:
(i) when the particle has a diameter on the submicron order or less, the measured particle size must be corrected by a light absorption constant corresponding to the refractive index and particle diameter thereof, PA1 (ii) the brownian motion and convection of the particles affect the measurement of the particle size, PA1 (iii) it takes a considerable time to measure the particle size, and PA1 (iv) reproducibility of the measured data and integrity between different types of devices are not so favorable.
A device using the electrozone method detects a variation in electric resistance that appears when particles floating in an electrolyte pass through an orifice. The device has the advantage that the volume-equivalent diameter of the particle can be measured with virtually no influence from the shape thereof. However, the device using the method has also the following drawbacks:
1) The range in which the diameter of particles can be measured with one kind of orifice is very narrow, since the particles that are smaller than the orifice remain undetected and since the particles that are larger than the orifice clog the orifice.
2) Since no accurate particle volume can be measured when the particles do not pass through the center of the orifice, no reliable particle size distribution can be obtained.
3) When the detection region is wide, the probability is high that two or more particles pass through the orifice at the same time. In this case, the method fails to provide an accurate particle volume and a reliable size particle distribution.
4) The electrozone method can measure a particle having a diameter larger than about 0.5 .mu.m. It is very difficult for the method to measure a small particle having a diameter less than 0.5 .mu.m.
A device using the currently available laser diffraction method calculates a particle size distribution based on the Mie's Scattering Theory from information on the angle distribution of the intensity of diffraction light and scatter light obtained by irradiating a suspended group of particles with a laser beam. This type of device has the advantage that only one time measurement provides reproducible data of particle size distribution with respect to particles having a diameter of 1 .mu.m to several hundred .mu.m. However, this type of device has the following drawbacks:
1) A difference in shape, refractive index, and surface state or the like largely affects an intensity of light scattered by particles, and it is very difficult, in particular, to determine an accurate particle size distribution with respect to particles having a size of submicron order.
2) This type of device requires a real refractive index of particle to be measured. An accurate particle size distribution cannot be obtained owing to oxidation of the particle surface and contamination of the particle when the refractive index given to the device is a normal value.
3) A particle size distribution is calculated on the assumption that the particle has a round shape and smooth surface but cannot be calculated when the assumption cannot be established.
4) The measurement results may be largely different between different types of devices because they differ in mitigation for the above drawbacks.
Furthermore, particle analyzers are known which primarily analyze a blood cell or a cell with a combination of the electrozone method and the laser diffraction method (for example, refer to Japanese Published Examined Patent Application No. HEI 2-25133, Japanese Published Unexamined Patent Application No. HEI 3-194444, and Japanese Published Examined Patent Application No. HEI 4-49903).
However, these combination type particle analyzers for analyzing only blood cells or cells analyze particles having a diameter large enough to be detected by both the electrozone method and the laser diffraction method in order to increase the amount of analysis information. They are not intended to measure sizes of much finer particles.
The electrozone method allows such analyzer to detect the particle having a diameter larger than about 0.5 .mu.m whereas the laser diffraction method allows such particle analyzer to detect the particle having a diameter larger than about 0.1 .mu.m.